In recent years, the drilling depth of an oil well or a gas well (hereinafter generically referred to as oil well) has tended to become deeper and deeper, and in order to increase collapse pressure of a casing or the like, an increase in strength of oil country tubular goods has been required.
Conventionally, as the oil country tubular goods, a seamless steel pipe or an electric resistance welded steel pipe has been used. In the case of a steel pipe requiring high strength, after the pipe forming, by performing quenching and tempering on the whole steel pipe, the strength has been secured and the toughness has been improved. On the other hand, recently, with the aim of reducing the drilling cost, a need has been increasing for an as-formed electric resistance welded steel pipe without being subjected to a heat treatment to be performed after the pipe forming.
Among the steel pipes for an oil well, a low-cost electric resistance welded steel pipe has been used for the case when one having relatively low strength is allowed, such as a casing located near the ground surface or the like. Further, the API specification Spec 5CT K55 electric resistance welded oil country tubular goods is manufactured in a state of being as-formed into a pipe (in a manner that quenching and tempering are omitted), and electric resistance welded oil country tubular goods corresponding to API specification Spec 5CT N80 is sometimes manufactured in a state of being as-formed into a pipe. However, in the case when the strength equal to or more than the above strength, for example, the strength corresponding to API specification 5CT P110 is needed, it has not been possible to perform the manufacture without performing a heat treatment after the pipe forming.
For example, in Patent Document 1 and Patent Document 2, there has been disclosed a technique in which after a steel pipe is made, only a weldheat-affected zone is reheated and then is quenched, and thereafter the whole steel pipe is tempered to thereby secure strength.
However, in the case of the electric resistance welded steel pipe, when the tempering is performed on the whole steel pipe, the manufacturing cost is increased. Further, when the quenching is performed, the dimensional accuracy is deteriorated by heat strain, and thus a process such as straightening is needed again.
For such a problem, there has been proposed a manufacturing technique of a high-strength steel pipe allowing a heat treatment for the whole steel pipe to be omitted.
For example, in Patent Document 3, there has been disclosed a method of manufacturing an electric resistance welded steel pipe whose strength and low temperature toughness are achieved not by performing a heat treatment for the whole steel pipe but by performing accelerated cooling after hot-rolling, coiling at a temperature of 250° C. or lower, and making a microstructure to be turned to a two-phase structure of MA constituent consisting of martensite and retained austenite mixed therein and microscopic ferrite.
Further, in Patent Document 4, there has been disclosed a method of manufacturing an electric resistance welded steel pipe whose strength is improved not by performing a heat treatment for the whole steel pipe but by performing rapid cooling after hot-rolling of a slab with high C content.